Definition of Stack

Stack is defined as a special type of data structure where items are inserted from one end called top of stack and items are deleted from the same end.

          Here, the last item inserted will be on top of stack. Since deletion is done from the same end, Last item is inserted is the First item to be deleted out from the stack and so, stack is also called Last In First Out (LIFO) data structure.

The various operations that can be performed on stacks are

         i.            Insert an item into the stack
       ii.            Delete an item from the stack
      iii.            Display the contents of the stack

·  Insert or push operation: Inserting an element in the stack is called push operation. This can be achieved by first increment top by 1 and then insert an item as shown below;
                top = top+1;
                s[top] = item;
These two statements can also be written as s[++top]= item
                             
When the stack is full the value of top will be [STACK SIZE -1] and it is not possible to insert any new item in the stack. This situation is called stack overflow.

·  Delete or Pop operation: Deleting the stack called pop operation. This can be achieved by first accessing the top element s[top] and then decremented top by one as shown below.

              Item = s[top--];

Each time, the item is deleted, top is decremented and finally, when the stack is empty the top will be -1. When the stack is empty, it is not possible to delete any item and this situation is called underflow of stack.

·      Display operation: Displaying the items of the stack is called display operation.

Applications of Stack:- A stack is very useful in situations when data have to be stored and then retrieved in the reverse order. Some applications of stack are listed below.
         i.            Function calls
       ii.            Large number Arithmetic
      iii.            Evaluation of arithmetic expressions

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